The Mg-xAl(x=9, 12, 15) alloys were prepared by meaning of semi-continuous casting. The effects of Al contents on evolution of microstructure of Mg-Al alloy were investigated used X-ray diffraction(XRD), optical microscopy(OM) and scanning electron microscopy(SEM). Meanwhile, the corrosion behaviors of Mg-xAl(x=9, 12, 15) alloys were studied by both hydrogen evolution and Potentiodynamic polarization curves. Moreover, the corrosion mechanism of Mg-xAl(x=9, 12, 15) alloys was also discussed in details based on the SEM corrosion morphologies of as-cast Mg-Al alloys after hydrogen evolution testing for 72 h in 3.5 wt.% NaCl without corrosion products. The results indicated that the microstructure of three alloys is composed of α-Mg phase and β-Mg₁₇Al₁₂ phase. Moreover, the content of beta phase increases with the increase of Al content. As a result, the corrosion resistance of the three alloys was enhanced with the increase of the β phase. The Mg-15%Al alloy has the best corrosion resistance, and Mg-9%Al is the worst. The oxide film formed on the all as cast Mg-Al alloys surface after hydrogen evolution testing for 72 h in 3.5 wt.% NaCl. Furthermore, the stability of the film increased with the increase of Al content.